The invention relates to magnetron power supplies. A magnetron is a thermionic vacuum tube which generates microwave power, for example microwave cooking power.
A magnetron tube has two concentric cylindrical electrodes. The inner electrode is the cathode and the outer electrode is the anode. The anode is divided into segments which form the walls of resonant cavities within the tube. In operation, electrons are extracted from the cathode and are accelerated toward the anode. By immersing the tube in a magnetic field and adjusting the voltage across the anode and the cathode, the electrons can be made to circulate around the cathode. The energy of the circulating electron cloud is then coupled into the cavities which are resonant at a microwave frequency. Microwave oscillation modes can then be extracted from the tube.
In order to produce the circulating electron cloud, the voltage across the anode and the cathode must be above a threshold value. The threshold value is the voltage above which the magnetron tube begins to conduct an electric current. Below the threshold, the circulating electrons never reach the anode. Instead, they spiral back toward the cathode under the influence of the forces created by the magnetic field.
There are two well known inherent related problems in connection with the operation of magnetrons. First, the magnetron anode current varies rapidly as a function of the anode voltage above the threshold value, and it is very difficult to control this current. In order to overcome this problem, most magnetron power supplies utilize a current limiting transformer design. (See, for example, U.S. Pat. No. 3,396,342.)
The second problem is concerned with the control of the output power of the magnetron. Although the magnetron is a nonlinear device, it behaves as a positive resistance in that an increase in the applied voltage produces an increase in the current flowing therethrough. However, because small changes in the applied voltage produce large variations in the current, voltage control of the magnetron power output is not practical.
While various methods and circuits have been proposed for controlling the average magnetron output power without controlling the voltage applied to the magnetron, the most often used method consists of cycling the entire magnetron power supply on and off. (See, for example, U.S. Pat. No. 4,220,841; United Kingdom Pat. No. 1,524,722; Wechsler, "Solid-state power control of microwave ovens," Appliance Engineer, December 1976.) During the "on" part of the cycle, the magnetron produces essentially 100% power output. The higher the ratio of "on"-time to "off"-time, the greater the average power output of the tube.
This method of controlling the magnetron power has at least two major disadvantages. First, in most power supplies the magnetron cathode heater is powered by a secondary winding on the main power transformer which supplies the high voltage to the magnetron's cathode. By repeatedly cutting off the power to the primary of the transformer, not only is the high voltage cycled between zero and the cathode operating voltage, but the tube filament is also repeatedly cooled down and cold started. Such a destructive process shortens the tube life due to cathode deterioration.
Second, the transient voltages induced in the high voltage power supply by cycling it on and off can greatly exceed the normal operating voltages, due to inductive effects. Consequently, the circuit components must be designed to withstand these much higher voltages, resulting in increased volume, weight, and cost for the magnetron power supply.
Other methods of controlling a magnetron's power output have also been proposed. For example, instead of placing a switch in the primary circuit of the power transformer, a switch can be placed in the high voltage secondary circuit (U.S. Pat. No. 4,220,841.) In this arrangement, the tube is no longer cooled down and cold started in each cycle. However, this method requires an expensive high voltage switch, expensive safety measures to insulate the operator of the device from high voltages, and expensive components to withstand the high voltage transients which are still present.
Finally, in another method the magnetron power output is controlled by systematically opening and closing the power circuit to the magnetron heater, while leaving the high voltage supply connected to the magnetron's cathode. (U.S. Pat. No. 4,220,841.) The problem with this method is, of course, that the tube is operated largely in the temperature-limited emission mode (rather than in the space-charge limited emission mode) which is destructive to the cathode and shortens the tube life.